Sea star invaders are local

Researchers from the Hawaiʻi Institute of Marine Biology take a non-lethal DNA biopsy samples from crown-of-thorns sea star (photo by D. Smith)

One of the greatest biological threats to tropical coral reefs can be a population outbreak of crown-of-thorns sea stars. Outbreaks can consume live corals over large areas, a change that can promote algal growth, alter reef fish populations and reduce the aesthetic value of coral reefs. Despite more than 30 years of research, the triggers and spread of crown-of-thorns outbreaks are not fully understood.

Human impacts such as urbanization, runoff and fishing have been correlated with outbreaks, but some outbreaks continue to occur in the absence of known anthropogenic triggers. Waves of a spreading outbreak that moves southerly along the Great Barrier Reef are termed secondary outbreaks because they are thought to be seeded from dispersing larvae of a primary outbreak upstream.

This secondary outbreak hypothesis has been widely accepted but a team of scientists from University of Hawaiʻi at Mānoa’s Hawaiʻi Institute of Marine Biology and the Joint Institute for Marine and Atmospheric Research and Rutgers University demonstrated that unlike on the Great Barrier Reef, crown-of-thorns larvae are not moving en masse among central Pacific archipelagos. In fact, crown-of-thorns outbreaks came from local populations.

On a finer scale, genetic differences were detected among reefs around islands and even between lagoon and forereef habitats indicating that the larvae of this species are not routinely reaching their full dispersal potential, and are not fueling outbreaks at distant sites.

Mānoa’s Christopher E. Bird, Derek J. Skillings, Molly A. Timmers and Robert J. Toone proved that crown-of-thorns outbreaks are not some rogue population that expands and ravages across central Pacific reefs. Instead, they hypothesize that nutrient inputs and favorable climatic and ecological conditions likely fuel outbreaks of local populations.

“The genetic differences found among crown-of-thorns populations clearly indicate that outbreaks are not spreading from the Hawaiian Archipelago to elsewhere. Furthermore, the similarity between outbreak and non-outbreak crown-of-thorns populations within each archipelago indicates that outbreaks are a local phenomenon,” explained Toonen.

“Our recommendation to managers is to seriously consider the role that environmental conditions and local nutrient inputs play in driving crown-of-thorns outbreaks,” said Toonen.

As far as we can tell, collection of ornamental fish does not impact the crown-of-thorns populations at all. Hawaii has fewer and smaller outbreaks than many places where there is less aquarium collecting, and there are outbreaks of COTs happening in places where there has never been aquarium collecting.

The data indicate that it is most likely nutrient enrichment (primarily from fertilization of the nearshore waters from terrestrial runoff around homes, golf courses and sewage outputs) that drive outbreaks because the nutrients feed algal blooms that in turn feed the larval sea stars.

An individual sea star can produce a million offspring and they may all survive if they have enough food.